There has been increasing concern over the security of access manholes for conduits containing the optic fibre cables used in telecommunications networks. Access for maintenance and repair is required at spaced locations along the conduit which is buried beneath the ground, but access to these conduits has to be restricted to authorised personnel. There is concern for example that terrorists may disable key routes in order to disrupt business and critical government communications, or that they may use manholes to place remotely controlled or timed explosive devices in open locations. By way of example, in urban environments, optical fibres often share the routes of copper telephone wires and the manholes are located every 150 metres or so in city duct structures: in the AT&T network in the USA, there are over 50,000 such manholes.
Security has conventionally been applied by the use of locking arrangements such as security bolts, requiring keys to open them, but this offers no security against a prepared intruder. Also, barriers tend to jam in icy conditions and are prone to dirt ingress.
The purpose of the invention is to provide an appropriate level of security governing manhole cover access, in a way which provides reliability and minimises installation cost. It is a further objective of the invention to allow such security to be provided with existing manhole cover installations.